A7ii7nals found in the Mitchehtown Cave. 33 



defective vision would become segregated in caves, since those 

 with the best eyesight, when carried in, would be most easily 

 able to find their way back to daylight. There is another 

 factor to be taken into account, that known as "economy of 

 nutrition." There is a general tendency among cave-animals 

 to a decrease in size, and their food supply is undoubtedly very 

 limited. Hence, to get rid of any useless organ would seem 

 to be of considerable benefit ; and this last factor would be 

 more likely to produce some effect among cave-dwellers than 

 among species living in the open air. 



A word as to the nourishment of these cave animals may be 

 of interest. Springtails live on vegetable refuse. The food 

 canal of every specimen of the Lipura which we collected 

 was filled with the fine red earth already mentioned as lining 

 the bottom of the cave. The species of Lipura are generally 

 found in damp mould, &c., and with this their intestines are 

 filled ; it appears therefore that they obtain their nutriment 

 from the vegetable substances contained in the mould. But 

 one would think that the amount of organic matter in the red 

 earth of the cave must be excessively small. No doubt it is, 

 but fragments of wood, &c., must be often carried in by water, 

 while some of the lower fungi find in the caverns a congenial 

 home. From the disintegration of these, diffused through 

 the earth, the insects have to win their precarious livelihood. 

 Dr. K. Perceval Wright has kindly told me that collectors in 

 the Carniolan caves secure insects b}^ leaving pieces of wood 

 as traps. The creatures must gladly leave their precarious 

 nourishment for the opportunity of such a feast. The spiders, 

 of course, live by prey, though it seems they can have but 

 little to ensnare. The habit of the family to which Porrhomma 

 wjj/^/5 belongs isto spin aweb of irregular, intercrossing threads. 

 If the cave-dwelling spider has maintained this habit, it 

 probably lives on insects which stray into its dark abode from 

 the outer world. But the fact that the specimens taken were 

 found walking on the rocks rather suggests that they hunt for 

 the cave-insects. Gamasid mites are stated by Mr. A. D. 

 Michaer to devour Lipuridae, so Gamasiis attemiatus must find 

 suitable food in the Mitchelstown cave. 



The apparent geographical distribution of the animals 

 demands a short consideration. The Lipura is hardly to be 



'^J'otirn, Linn, Soc. {Zool^ vol. xv,, 1881. 



A3 



